[WIP] Add Stop funcs to retrieval providers

[shannonwells]

Change summary

• Add a Stop func to interface and implementation for retrieval provider

Effects:
if a Provider is still connected to the libp2p network and a request comes in, the response will be an error: protocol mismatch in lazy handshake.

[codecov-io]

Codecov Report

Merging #56 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #56     +/-   ##
=========================================
+ Coverage   33.82%   34.12%   +0.3%     
=========================================
  Files          36       36             
  Lines        2608     2615      +7     
=========================================
+ Hits          882      892     +10     
+ Misses       1590     1588      -2     
+ Partials      136      135      -1

Impacted Files	Coverage Δ
retrievalmarket/network/libp2p_impl.go	74.36% <100%> (+12.6%)	⬆️
retrievalmarket/impl/provider.go	72.8% <100%> (+0.45%)	⬆️

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[laser]

@shannonwells - Looks like a sane PR to me.

[hannahhoward]

LGTM. Why are we changing the context timeouts?

[laser]

@shannonwells - PR is ready to move forward, modulo test failure